Demonstration that a behavior aggregates in families is the first step in establishing a genetic basis for a disorder (Hewitt & Turner 1995). Decreasing similarity with decreasing degrees of relatedness lends support to genetic influence on a behavior (Gottesman 1991). This is a necessary, but not sufficient, condition for heritability. Similarity among family members is due both to shared genes and shared environment; family studies cannot tease apart these two sources of variance to determine whether familiality is due to genetic or common environmental causes (Sherman et al. 1997). However, family studies provide a powerful method for identifying gene-environment interaction. By comparing high-risk children, identified as such by the presence of psy-chopathology in their parents, with a control group of low-risk individuals, it is possible to test the effects of environmental characteristics on individuals varying in genetic risk (Cannon et al. 1990). In a high-risk study of Danish children with schizophrenic mothers and matched controls, institutional rearing was associated with an elevated risk of schizophrenia only among those children with a genetic predisposition (Cannon et al. 1990). When these subjects
